Theory of (<i>s + id</i>) pairing in mixed-valent correlated metals

Motivated by the recent discovery of superconductivity in square-planar nickelates as well as by longstanding puzzling experiments in heavy-fermion superconductors, we study Cooper pairing between correlated d-electrons coupled to a band of weakly-correlated electrons. We perform self-consistent large N calculations on an effective t - J model for the d-electrons with additional hybridization. Unlike previous studies of mixed-valent systems, we focus on parameter regimes where both hybridized bands are relevant to determining the pairing
symmetry. For sufficiently strong hybridization, we find a robust s + id pairing which breaks time-reversal and point-group symmetries in the mixed-valent regime. Our results illustrate how intrinsically multi-band systems such as heavy-fermions can support a number of highly non-trivial pairing states. They also provide a putative microscopic realization of previous phenomenological proposals of s+id pairing and suggest a potential resolution to a number of puzzling experiments in heavy-fermion superconductors.